The present invention relates in general to a method of manufacturing a semiconductor device and in particular to a method of selectively diffusing aluminium into a single crystal silicon substrate.
In general, there are known boron, aluminium, gallium, indium or the like for P-type dopants or impurities diffused in a single crystal silicon substrate. Among those elements, aluminium is the most useful dopant for forming a very deep diffusion layer in the silicon substrate, because aluminium exhibits the greatest diffusion coefficient upon being diffused into the silicon substrate. In effect, the diffusion coefficient of aluminium is ten or more times as great as that of boron and several times as great as that of gallium. Besides, the time required for diffusing aluminium in forming a diffusion layer of a predetermined depth amounts to only one-tenth or less of that required for the diffusion of boron and only a fragment of the time required for the diffusion of gallium. Additionally, aluminium is advantageous in respect of the fact that generation of distortions in the crystal lattice of silicon can be suppressed to a minimum and that interaction with other dopants is negligible. On the other hand, diffusion of aluminium into silicon brings about various difficulties in constructing a diffusion processing apparatus. Further, the masking effect of a silicon dioxide film can not be satisfactorily attained in the case of the diffusion of aluminium.
Attempts have been hitherto made to use a selective diffusion mask of materials other than silicon dioxide for the selective diffusion of aluminium into silicon. However, such attempts have not yet led to success. For the mask materials, there have been proposed a silicon nitride film, a metal oxide film such as alumina, semiconductor films or a composite film of these materials. However, such masking materials encounter difficulties in the manufacture of the desired mask and in the photo-etching process. Additionally, pin-holes and cracks are likely to be produced in the mask during the diffusion and aluminium is thus diffused through them. Further, aluminium tends to react with the mask material, thereby to make the after-treatment difficult. For these reasons, the hitherto proposed masks are not easily used for practical applications.
There is also known a method of doping a silicon substrate with aluminium from dots or buttons of aluminium or alloys thereof deposited on a surface of the silicon substrate. This process is referred to also as a temperature gradient zone melting method and has been developed from the technique of forming an alloy junction, which method is not thus inherently the selective diffusion method. At present, it is still impossible to control satisfactorily the boundary pattern between the regions to be doped with aluminium and those not to be doped as well as the concentration profile of the dopant.